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PHYS 314 > Harmonics > Flashcards

Harmonics Flashcards

1
Q

What does harmonics equal?

A

Two times the operating frequency

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2
Q

What is another name for harmonics and what is harmonics a result of?

A

Beam dynamics

Results from propagation through tissue

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3
Q

What is wave distortion?

A

As the ultrasound beam propagates several harmonic frequencies are produced as a result.

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4
Q

As density increases, velocity will do what?

A

Decrease

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5
Q

As compressibility increases velocity will do what?

A

Increase

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6
Q

What is wave distortion dependent on? (3)

A
  • Intensity of the beam
  • Distance travelled
  • Nature of the tissue
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7
Q

How are harmonic frequencies produced?

A

Non-linear (a little different when they come back each time)

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8
Q

How are fundamental waves produced?

A

Linear

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9
Q

How does high pressure affect compression and velocity?

A

Higher pressure causes higher compression and therefore increased velocity

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10
Q

How does low pressure affect compression and velocity?

A

Lower pressure causes the tissue to expand (less compressed) and therefore the velocity is lower.

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11
Q

Harmonics can benefit the image in what three ways?

A
  • Narrower beam (harmonics are best produced from most intense part of beam)
  • Eliminated grating lobes (grating lobes are too weak to produce harmonics)
  • Reduced/eliminated reverberation
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12
Q

What was developed to eliminate the fundamental frequency and allow the harmonic signal to pass through the beam former

A

Bandpass filtration

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13
Q

When we say that the fundamental freqeuncy was eliminated with bandpass filtration, what can we actually think of?

A

Having a filter, the fundamental frequencies are not allowed through the beam former, only the harmonic ones

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14
Q

For bandpass filtering to work, the bandwidths must do what?

A

The fundamental (Fo) and second harmonic (2Fo) bandwidths must fit within the overall transducer bandwidth without overlapping (more narrow bandwidths)

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15
Q

Narrow bandwidth means what in terms of pulse and axial resolution?

A

Longer pulse and poor axial resolution

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16
Q

What is a technique used to filter out the fundamental frequency and leave only the harmonic signal while maintaining a wide bandwidth?

A

Pulse inversion

17
Q

What is pulse inversion?

A

A pulse is sent out followed by a second inverse pulse allowing filtration of the fundamental frequency while maintaining a wide bandwidth.

18
Q

How does pulse inversion work?

A

Fundamental frequencies are cancelled out when they are added together due to being linear, while harmonic frequencies prevail due to being non-linear (distorted).

19
Q

What are contrast agents?

A

Liquid suspensions that can be injected into the blood stream to improve the return of echoes for enhanced visualization of blood/tissue.

20
Q

What are three musts for a contrast agent to be successful?

A
  • Good back scatter
  • Have to be small enough to pass through capillaries but large enough to produce echoes
  • Have to be strong/stable enough to pass through the heart for several cycles to allow adequate imaging time
21
Q

What are the majority of contrast agents made of?

A

Microbubbles of gas contained within a protein, lipid, or polymer shell

22
Q

What is coded excitation?

A

A machine program that allows the visualization of blood flow in 2D without doppler (avoids associated artifacts) by digitizing ultrasound pulses to help reduce noise and improve overall image quality.

23
Q

What kind of frame rate does coded excitation give you?

A

Higher frame rate

24
Q

What enhancements does coded pulse allow? (7)

A
  • Higher frame rate
  • Increases sensitivity (ability to pick up weak echoes)
  • Suppresses noise
  • Improved spatial resolution/small lesion identification
  • Improved penetration
  • Improved contrast/subtle contrast difference detection
  • Helps with TDS patients (improved productivity, better IQ and probe versatility)
25
Q

What does coded excitation do for penetration and contrast?

A

Improves it (improves subtle contrast differences)

26
Q

What is the purpose of the gas and the shell in microbubble contrast?

A

The gas creates a large impedance mismatch (scatter)and the shell keeps the gas from dissolving into the blood

27
Q

Why are contrast agents useful?

A

They can improve lesion detection and increase doppler signals

28
Q

What happens to microbubble contrast when scanning with 100% power?

A

The bubbles collapse and the gas is dissolved

29
Q

How is microbubble affected by 0% output power?

A

It is static

30
Q

What is the ideal output power when scanning microbubble contrast and why?

A

75%

The bubbles will produce very intense harmonic echoes which increase the contrast between the contrast and the tissue

31
Q

How does increasing power affect microbubble contrast?

A

0% = Static
0 - 50 = Resonance
50 - 100 = Harmonic production
100% = Implosion

32
Q

What mode does coded excitation use?

A

B-mode

33
Q

How does coded excitation work?

A
  1. Digital encoder codes the pulses so the weak blood echoes can be differentiated from noise
  2. Decoder enhances the flow signal
  3. Flow and tissue displayed in B-mode
34
Q

How does coded excitation differ from traditional colour doppler?

A
  1. CE gives simultaneous tissue and flow info unlike the transposed overlay of doppler onto 2D image
  2. Intuitive B-mode display gives full field of view
  3. No separate firings for flow allows a higher frame rate

PHYS 314 (5 decks)

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